1. Field of Invention
This invention relates to a method for fabricating a photodiode image sensor device. More particularly, the present invention relates to a method for fabricating a CMOS image sensor (CIS).
2. Description of Related Art
A photodiode image sensor device is the most commonly used device for detecting images. A typical photodiode image sensor device comprises a reset transistor and a light sensor region formed by a photodiode. For example, a photodiode is formed with an N type doped region and a P type substrate. When the photodiode image sensor is in operation, a voltage is applied to the reset transistor gate to turn on the reset transistor and to charge the N/P diode junction capacitor. The reset transistor is turned off when the charging of the N/P diode junction capacitor has reached a certain high voltage. The N/P diode generates a reverse bias to form a depletion region. When a light is shinned on the NIP diode light sensor, electrons and holes are generated. These holes and electrons are separated by the electrical field of the depletion region, causing the electrons to travel in the direction of the N-type doped region to lower the voltage of the N-type doped region, whereas the holes travel in the direction of the P-type substrate.
A charge coupled device (CCD) has a high dynamic range and a low dark current. The sophistication of the current technology of a charge coupled device allows the charged couple device to become the most popular image sensing device. The manufacturing for a charge coupled device is, however, rather special. The price for a CCD is therefore very high. Moreover, the driver requires a high voltage operation, leading to the problems of high power dissipation and inability of random access of memory.
A CMOS image sensor has the characteristics of high quantum efficiency, low read noise, high dynamic range and random access, and it is one hundred percent compatible with the manufacturing for a CMOS device. A CMOS image sensor can combine with other control circuit, A/D converter and several signal processing circuits on a single wafer to achieve the so-called system on a chip (SOC). The progress of the technology of a CMOS image sensor, therefore, greatly reduces the cost of an image sensor device, the picture size and the power of dissipation. The CMOS image sensor is therefore slowly replacing the charge coupled device.
The manufacturing method for a conventional CMOS image sensor is summarized in the following.
Referring to FIG. 1A, a field oxide layer 102 is formed on a substrate 100. A reset transistor 120 that comprises a gate oxide layer 104 and a polysilicon gate 106 is formed on the substrate 100. Using the field oxide layer 102 and the polysilicon gate 106 as implantation masks, ion implantation and thermal drive-in processes are conducted to form the source/drain region 108 and the doped region 112 of the photodiode sensor region 110. A spacer 114 is formed on the sidewalls of the polysilicon gate 106 and the gate oxide layer 104. A self-aligned block (SAB) 116 is further formed on the photodiode sensor region 110 to complete the formation for a CMOS image sensor device.
The conventional manufacturing method for a CMOS image sensor, however, has the following problems.
After the completion of the manufacturing of the above CMOS image sensor, the backend process is conducted, such as the formation of the inter-layer dielectrics and metal conductive line, which are used for the controlling of the device. The application of plasma etching is inevitable in the backend process for, for example, the defining of the contact/via opening or the metal conductive line. The high power plasma, however, can penetrate the inter-layer dielectrics to induce damages on the surface of the photodiode. The damages inflicted upon the surface of the photodiode due to plasma etching are especially prominent in the vicinity of the bird""s peak region. As a consequence, current leakage occurs more easily in the photodiode sensor region. The aforementioned current leakage problem would cause the CMOS image sensor to generate a significant dark current, leading to an increase of read noise.
The present invention provides a fabrication method for a CMOS image sensor, wherein a protective layer is formed on the CMOS image sensor before the backend process to prevent the CMOS image sensor from being damaged by plasma.
The present invention provides a fabrication method for a CMOS image sensor, wherein the dark current problem of the CMOS image sensor is greatly mitigated.
The present invention provides a fabrication method for a CMOS image sensor, wherein an isolation layer is formed in the substrate to isolate the photodiode sensor region and the transistor device region. Thereafter, a gate structure is formed on the transistor device region, followed by performing a light ion implantation process to form a lightly doped drain region of the transistor device and to form a lightly doped region of the photodiode sensor region. A spacer is further formed on the sidewall of the gate structure, followed by performing a heavily doped implantation process to form a source/drain region in the transistor device region and a heavily doped region in the photodiode sensor region. After this, a self-aligned block is formed on the photodiode sensor region, followed by forming a protective layer to cover the entire substrate, wherein the protective layer and the self-aligned block comprise different refractive indices.
Accordingly, an aspect of the present invention is forming a protective layer to cover the entire substrate after the manufacturing of the CMOS sensory device. The photodiode sensor region is thus protected from being damaged during the subsequent backend process to minimize the generation of dark current.
Moreover, besides protecting the photodiode sensory region, the protective layer formed on the entire substrate also protects other regions from being damaged by plasma etching.
Additionally, the protective layer and the self-aligned block comprise different refraction indices. As the incident light penetrates the surface of the photodiode sensor region, the incident light is refracted by the protective layer and the self-aligned block, which are of different refraction indices. The convertibility into photoelectrons of the light absorbed by the photodiode, after being refracted by the two layers of different refraction indices, is better. In another words, quantum efficiency is higher.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.